The present invention relates to techniques for preventing acoustic inconveniences in acoustic spaces.
In an acoustic space, such as a hall or theater, surrounded by walls, acoustic inconveniences, such as booming and flatter echoes, may occur by sounds being repeatedly reflected between the walls opposed parallel to each other. FIG. 10 is a front view of a conventionally-known acoustic structure 50 suited to prevent the above-mentioned acoustic inconveniences. The conventionally-known acoustic structure 50 comprises a plurality of rectangular cross-sectional pipes 51-j (j=1-7) of different lengths arranged in parallel relation to one another so as to define a flat surface as a whole. Further, each of the rectangular cross-sectional pipes 51-j (j=1-7) is formed of a reflective material having a high rigidity. Further, the rectangular cross-sectional pipes 51-j (j=1-7) have respective opening portions 52-j (j=1-7) that are oriented (or open) in a same direction. The acoustic structure 50 is installed on an inner wall, ceiling, or the like with the openings 52-j (j=1-7) of the pipes 51-j (j=1-7) oriented toward the middle of an acoustic space.
In the thus-constructed acoustic structure, each of the pipes 51-j (j=1-7) resonates in response to sound waves of a particular resonance frequency of sound waves falling from the acoustic space in the individual opening portions 52-j (j=1-7). Because of such resonance, sound waves radiated from interior hollow regions of the pipes 51-j (j=1-7) to the acoustic space via the opening portions 52-j (j=1-7) produce sound absorbing and sound scattering effects near the opening portions 52-j (j=1-7). As a consequence, sound waves propagated from the acoustic space toward the pipes 51-j (j=1-7) are dissipated in the pipes 51-j (j=1-7), so that occurrence of acoustic inconveniences can be prevented. An example of this type of acoustic structure 50 is disclosed in Japanese Patent Application Laid-open Publication No. 2002-30744 (patent literature 1).
In the aforementioned type of acoustic structure 50, the sound absorbing and sound scattering effects are produced at resonant frequencies determined by respective constructions of the pipes 51-j (j=1-7). Each of the pipes 51-j (j=1-7) has not only a fundamental resonance mode but also a high-order resonance mode. Thus, the acoustic structure 50 can achieve sound absorbing and sound scattering effects over wide frequency bands by causing each of the pipes 51-j (j=1-7) to resonate not only in the fundamental resonance mode but also in the high-order resonance mode.
Actually, however, with the pipes 51-j of the acoustic structure 50, sound absorbing and sound scattering effects produced in response to sound waves of high frequency bands, particularly in a range of 2 kHz-4 kHz, entering or falling in the opening portions 52-j are smaller than sound absorbing and sound scattering effects produced in response to sound waves of low frequency bands falling in the opening portions 52-j (j=1-7). Thus, when sound waves of high frequency bands have been produced in the acoustic space, acoustic energy of the produced sound waves cannot be dissipated sufficiently with the pipes 51-j.